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View Full Version : Requirements and limits for life in the context of exoplanets



A.DIM
2014-Jun-12, 03:25 PM
HERE's (http://www.pnas.org/content/early/2014/06/04/1304212111.full.pdf) a good overview how astrobiologists consider life in space.

Abstract: The requirements for life on Earth, its elemental composition, and its environmental limits provide a way to assess the habitability of exoplanets. Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life can grow and reproduce at temperatures as low as −15 C, and as high as 122 C. Studies of life in extreme deserts show that on a dry world, even a small amount of rain, fog, snow, and even atmospheric humidity can be adequate for photosynthetic production producing a small but detectable microbial community. Life is able to use light at levels less than 10−5 of the solar flux at Earth. UV or ionizing radiation can be tolerated by many microorganisms at very high levels and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen may limit habitability. Levels of O2 over a few percent on an exoplanet would be consistent with the presence of multicellular organisms and high levels of O2 on Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and salinity are likely to vary and not limit life over an entire planet or moon.


Of particular note to me is the nod to panspermia on pg 5: ... "If such dust grains or rocks were incorporated into the protoplanetary nebula, then every planet and moon that formed would be infected with life."

If

eburacum45
2014-Jun-12, 08:37 PM
"If such dust grains or rocks were incorporated into the protoplanetary nebula, then every planet and moon that formed would be infected with life."



This is an intriguing prospect, although the infection rate would be infinitesimally low unless the infection could propogate by self-replication within the cloud itself. This might be a bit problematic, since a protostar emits some very nasty radiation during its formation.


see http://en.wikipedia.org/wiki/T_Tauri_star

There is evidence of large areas of starspot coverage, and they have intense and variable X-ray and radio emissions (approximately 1000 times that of the Sun). Many have extremely powerful stellar winds. Another source of brightness variability are clumps (protoplanets and planetesimals) in the disk surrounding T Tauri stars. Maybe the outermost regions of the nebula would be a safer location for propogation than the innermost regions.